Most Waste to Energy (“WTE”) facilities in the United States were built 20 plus years ago, when commodities were not as valuable as they are today. As such they were not built with a particular emphasis in maximizing the recovery of ferrous and non-ferrous metallics present in the bottom ash, nor were they built with an emphasis of minimizing the bonding agents required to treat the bottom ash for soluble metallic content, such as lime and concrete.
The municipal solid waste “MSW” processed by these facilities typically contains around 1% non-ferrous metals, such as aluminum, copper, zinc, and lead. These metals have comparably low melting points and can melt when exposed to the high temperatures present in a furnace. Melting non-ferrous metals can result in the formation of tiny droplets of metal that separate from the larger body in the form of small sand-sized particles. This results in larger recoverable non-ferrous, and smaller more difficult to extract non-ferrous particles being present throughout the bottom ash.
Under traditional processing methods used at approximately 90 WTE facilities in the United States, it is the presence of water-soluble contaminants, including non-ferrous metals that results in the need to treat bottom ash to mitigate the potential of these metals leaching into the ground water.
Typically, as MSW is incinerated, the corresponding ash is squelched in water to mitigate fire hazard and smoke emissions. When the ash is squelched in water it tends to bind in a cementatious mixture of heterogeneous particle distribution that can retain as much as 40% moisture content. As such, it is the prevailing practice in the United States that if any attempt is made to segregate ash prior to treatment with a bonding agent, it is only the largest debris, such as that that could be removed by a rough sizing mechanism, such as a grizzly screen, a trommel, or a vibrating deck, that is removed prior to treatment. The large debris separated in this manner typically accounts for less than 10% of the bottom ash generated, and is typically the only fraction not treated with a bonding process.
Furthermore, typically only the largest WTE facilities attempt large-scale removal of non-ferrous metals from ash. For example, in the Western Continental United States there are six large incinerators producing more than 400,000 tons of bottom ash per year, none of which incorporate the use of eddy currents for non-ferrous extraction. In addition, most WTE facilities only attempt to recover metallics from the large debris. This lack of metallic recovery is not limited to the West, according to the Earth Engineering Center at Columbia University, approximately 51% of available ferrous metal, and over 92% of available non-ferrous metals are currently landfilled by existing WTE facilities in the United States, see FIGS. 5 & 6.
Even if more magnets and eddy currents were to be used, under the traditional process, they would have limited effectiveness due to the cement like property of wet bottom ash, which adheres to tramp metal like glue, reducing the recoverability of such metals, and down-grading its value if it were to be recovered under traditional methods.
As the volume of metals present in bottom ash, particularly soluble non-ferrous metals, can pose an environmental hazard, it is the prevailing practice in the United States to treat everything but the largest debris with a bonding agent, such as lime or cement, rather than removing or isolating the leachable metallics for treatment.
The practice of treating and disposing of a facility's bottom ash is expensive. A large WTE facility can spend millions of dollars a year treating its bottom ash and hauling it to a special landfill or monofill for disposal. The method proposed herein can increase the volume of debris in bottom ash not treated with a bonding agent from 35% to as much as 100%, thereby greatly reducing processing and disposal costs typically associated with WTE bottom ash.